GLOSSARY

Veins are vessels that return blood from the tissues to the heart; all veins except the pulmonary veins (and the umbilical vein in the foetus) carry oxygen-poor blood.

Characteristics

• Thinner walls than arteries with less smooth muscle; they collapse when empty.
• Larger lumen.
• Low pressure: about 0–5 mmHg in the large veins near the heart.
• Venous valves: especially in the lower limbs.
• Capacitance vessels: hold about 60–70 percent of the blood volume.

Types